It is known that a primary requirement relating to wires of this type, in addition to excellent mechanical characteristics, is good resistance to hydrogen embrittlement in an acid environment where sulfur is present, in particular in the form of H2S which is present in the fluids and hydrocarbons being transported.
The steel wires currently commercially available for offshore utilization are principally made of the low-alloy grades that have a tensile strength Rm of approximately 800 MPa.
To fabricate these cold rolled wires in the known manner, manganese steels comprising from 0.15 to 0.80% by weight carbon are used, the initial microstructure of which is ferritic-pearlitic. After shaping of the initial round rolled wire rod, an appropriate stress-relief heat treatment is applied to achieve the required hardness. However, the cold rolled wires obtained by these conventional processes cannot withstand the relatively severe acidity encountered in deep waters, due to the strong presence of H2S in the hydrocarbons transported.
In addition, flexible offshore conduits must currently be appropriate for use at increasingly greater underwater depths, which requires an increase in the mechanical breaking strength beyond 800 MPa as well as corrosion-fatigue strength enabling them to resist corrosion generated by the presence of H2S and CO2.
In addition, market constraints regarding prices are becoming increasingly tight, which has an adverse effect on the customary use of noble alloy elements such as chromium, niobium etc. or long or multiple and therefore expensive processing steps, above all if they must be carried out under hot conditions.